1. Field of the Invention
This invention relates to a holographic display system, and specifically relates to, for example, a holographic head-up display system.
2. Description of the Prior Art
Systems commonly used in aircraft to display images of instruments in the field of view of a pilot are termed "head-up displays". In general, the head-up displays employ optical elements or holographic elements to project an image of an instrument into the pilot's field of view as the pilot looks through the forward window of the aircraft.
U.S. Pat. No. 4,261,647 and U.S. Pat. No. 4,655,540 disclose such head-up displays.
U.S. Pat. No. 4,613,200 discloses a head-up display system usable in an automotive vehicle. Specifically, the head-up display system of U.S. Pat. No. 4,613,200 utilizes two parallel holographic optical elements to project a virtual image of an indicia display source. The virtual image is superimposed in the driver's field of view as the driver looks at the forward scene through the vehicle windshield. The parallel arrangement of the holographic optical elements ensures that the virtual image is free of chromatic aberrations.
It was experimentally found that, in a head-up display system which had two parallel holographic optical elements as disclosed in U.S. Pat. No. 4,613,200, there were a desired virtual image and also unwanted virtual images of an indicia display source in the driver's field of view. The unwanted images extended near the desired image or partially overlapped the desired image.